The present invention pertains to the field of seismic data acquisition and processing. More particularly the present invention relates to techniques and systems for attenuating noise in data collected using vibroseis acquisition methods.
In current common practice, seismic surveys using vibroseis sources are carried out using time-separated vibratory sweeps. The delay between one sweep finishing and the next sweep starting is at least the listening time of the survey. If this delay is not present then energy from one sweep in the form of harmonics can appear on the preceding shot record as noise. Thus, according to conventional practice there is a time delay between sweeps that is at least the sweep time plus the listening time. This delay time has been found to contribute significantly to the cost of the survey. Accordingly, a method to effectively reduce the delay time, without the detrimental effects of increased noise could result in significant cost savings.
Recently a technique known as slip-sweep acquisition has been developed as a method for speeding up seismic acquisition. By allowing sweeps to overlap in time, but ensuring that at least the listening time elapses between sweep start times, the seismic records are still separated in time after correlation. In some early tests of the technique in Oman, it was found that harmonic noise from the subsequent sweep did not result in appreciable noise on the final seismic image when multiple traces were summed at the post-migration stack.
See, e.g. Rozemond J, (1996) xe2x80x9cSlip-sweep acquisitionxe2x80x9d, in Technical Abstracts of the 66thAnnual meeting of the SEG, Nov. 10-16th, ACQ 3.2. Since the harmonic noise adds together incoherently and the signal component adds coherently, in that case visible signs of harmonic noise were apparently removed by the summation process.
However, it has been found from experimental tests performed where the near surface is very different to Oman, that appreciable harmonic energy can carry forward to the final stacked image. This noise can be visually very disturbing as it can have a very different frequency content than the signal. Typically, the harmonic noise that appears late in the record has more high frequencies present than the signal. In a normal wiggle display, this changes the visual character of the record substantially.
Thus, slip-sweep acquisition has the great advantage of reducing the overall time necessary to complete a fixed number of sweeps. One major problem, however, is the noise introduced from one or more of the subsequent sweeps.
A technique of slip-sweep acquisition is described in Wams, J. and Rozemond J. (1998), Recent Developments in 3-D acquisition using vibroseis in Oman, Leading Edge 17 no 8, pp 1053-1063, and also in Rozemond (1996). In Rozemond (1996), a technique is mentioned in which random variations in the delay time are introduced in an effort to eliminate noise from the overlapping sweeps. However, using this technique alone has been found to only be of limited effectiveness.
Another attempt at providing an acquisition method with reduced delay time allows sweeps to follow one another without the listening time delay. See, U.S. Pat. No. 5,410,517. This technique relies on phase cancellation to reduce noise from other sweeps. However, this technique is of only limited effectiveness and it suffers from the disadvantage that, in order to address both the 2nd and 3rd order harmonics, at least 4 sweeps per location are required. Performing additional sweeps is particularly problematic since it substantially increases the time needed and cost to perform the data acquisition.
Thus, it is an objective the present invention to provide a method of slip-sweep data acquisition which has a substantial reduction in noise due to overlapping sweeps while having little or no impact on the time required to perform the data acquisition.
According to a preferred embodiment of the invention, a method and apparatus is provided for reducing the effects of noise in seismic data acquired from overlapping vibratory sweeps. In particular, raw data traces are received which represent sensor recordings of earth vibrations caused by vibratory sweeps. The raw data also contains some noise caused by harmonic energy from one or more other, overlapping vibratory sweeps. The harmonic noise energy is caused by a subsequent vibratory sweep that begins while the initial sweep is still underway.
The data are recorded and stored as plurality of raw data traces representing received vibrations caused by the vibratory sweep and at least some noise from the subsequent vibratory sweeps. The raw data traces are processed such that reflections from substantially the same spatial position occur at substantially the same point in time. The data is digitally transformed preferably using a mirror filtering technique to generate a number of reduced bandwidth traces in the time frequency domain. The reduced bandwidth traces are mathematically equivalent to the time domain data, and each reduced-bandwidth trace represents a subset of the frequencies present in the time domain data. Noise estimates are calculated based on a running average over a predetermined portion of the reduced-bandwidth trace. The reduced-bandwidth traces are then summed and normalized according to the noise estimates.
Finally, the summed and normalized data is digitally transforming back into the time domain, generating a reduced noise data trace which is substantially a mathematical equivalent to the summed and weighted trace. The reduced noise data trace has less noise caused by the second vibratory sweep than original data trace, and it is therefore more suitable for further processing and analysis to determine therefrom characteristics of an earth structure.
According to an another preferred embodiment of the invention, a method provided for reducing the effects of noise in seismic data acquired from overlapping vibratory sweeps in which after processing the raw data traces, the seismic time domain data are convolved to generate transformed time domain data having the harmonic noise concentrated in time. The convolution is preferably performed with a synthetic sweep. Noise estimates are calculated, and the transformed data traces are summed and normalized according to the noise estimates. Finally, the summed and normalized data is deconvolved to generate reduced noise time domain data which has generally less harmonic noise caused by the second vibratory sweep than said first time domain data.